1. Field of the Invention
The present invention relates to an outboard motor provided with a water pump arranged to draw in cooling water.
2. Description of the Related Art
In a conventional outboard motor, outside water is drawn in by a water pump to be used as cooling water for an engine. A vane-type water pump is commonly used in which an impeller formed with a flexible material such as rubber is eccentrically rotated in a cylindrical pump housing so as to avoid negative influences of sand, impurities and the like that are frequently contained in the outside water.
In a conventional outboard motor, the water pump is disposed near a midsection of a drive shaft that transmits engine output to a propeller. The water pump is directly driven by the drive shaft (see JP-B-3509171).
However, as described above, since the water pump is directly driven by the drive shaft which rotates at the same speed as the engine, the rotational speed of the water pump becomes equal to that of the engine. Thus, when an outer diameter of the impeller is large, a speed at the tip of an impeller blade becomes excessively high at an engine speed of about 6,000 rpm, which is close to the maximum engine speed. Consequently, it causes cavitation in the pump housing and a plateau in the fluid discharge amount.